The Apicomplexa comprise a medically important phylum of obligate intracellular parasites, including Plasmodium falciparum, the causative agent of malaria, Cryptosporidium, which causes gastrointestinal disease, and the category B agent Toxoplasma gondii, which can be pathogenic in immunocompromised hosts and the developing fetus. Drug therapies that effectively treat all stages of some of these parasites are currently lacking and in many cases existing drugs are quickly becoming ineffective due to the emergence of drug-resistant parasite strains. Thus, the discovery of new targets for drug design and the understanding of resistance mechanisms are high priorities in the studies of apicomplexan parasites. Our approach to this challenge has been to isolate and characterize T. gondii mutants that are resistant to specific anti- parasitic drugs. We have recently discovered that disruption of a mitochondrial MutS homologue (MSH), TgMSH-1, directly confers multi-drug resistance in T. gondii. MSHs are critical components of the eukaryotic DNA mismatch repair machinery and are also involved in signaling cell cycle arrest and apoptosis in response to DNA damaging agents. Interestingly, we have observed that certain anti-parasitic drugs cause disruption in the expression of cell cycle markers in a TgMSH1 dependent manner. Thus, we have identified a novel pathway in T. gondii, that when induced by certain drugs leads to parasite death. It is our hypothesis that that certain drugs affect the mitochondrion of the parasite directly or indirectly and that this effect results in the activation of a signaling pathway, which includes TgMSH1 and results in parasite death. It is our goal to dissect this TgMSH1 dependent death mechanism as to characterize a novel mode of killing apicomplexan parasites. Using a combination of cell biology, biochemistry and genomic approaches we will determine the effect of TgMSH1-dependent drugs on mitochondrial function, identify signaling partners of TgMHS1 and determine the effect of TgMSH1 dependent drugs on cell cycle. The completion of these aims will elucidate the details and mechanisms of an inducible death pathway. This constitutes an innovative approach to the discovery of drug targets in this important pathogen.